Sun poisoning is an allergic response to the ultraviolet rays of the sun. A mild form results in sunburn while a more severe form causes vesicles or blisters under the skin, and a red, itchy rash in patches on the skin. Some medications can increase your sensitivity to the sun so check with your pharmacist regarding any medications you are taking. The best treatment is prevention: use of sunscreen with an SPF of 15 or greater.